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Chemical Nomenclature for Organic Compounds

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we will learn how to name organic compounds using chemical nomenclature. We will learn how knowing the name of a compound also helps us know the structure of the compound.

Chemical Names

Everything has a name, otherwise we wouldn't have a way to refer to it. But some things have several names. Chemicals frequently have more than one name. For example vinegar is diluted acetic acid, but this chemical acetic acid also has another name - ethanoic acid. The name 'ethanoic acid' is the chemical nomenclature for vinegar.

Chemical nomenclature is a specific method used to name chemicals. This method is set up in such a way that even if you had no idea what the structure of a chemical was, you could figure it out based on the name. Vinegar and acetic acid are common names used, but they don't tell you anything about the structure of the compound. 'Ethanoic acid' tells you the exact structure.

Ethanoic Acid Structure and Name

So let's look at the structure of ethanoic acid and explain how the name 'ethanoic acid' describes the structure:

acetic acid

So ethanoic acid has two carbon atoms, one of these carbon atoms is double bonded to an oxygen with a single bond to an oxygen-hydrogen. When carbon is bonded to two oxygen atoms in this way this entire group is called a 'carboxylic acid'. So ethanoic acid is a 2 carbon chain beginning with a carboxylic acid.

Identify COOH

Now let's look at the name. Ethan- refers to 2 carbons while -oic acid refers to a carboxylic acid. So the names tells us that ethanoic acid is a 2 carbon chain with a carboxylic acid. This same system works for all chemicals names using chemical nomenclature.

Number of Carbon Atoms

In naming organic chemical compounds the first thing we need to look for is the longest carbon chain, or the parent chain. Each number has a name associated with it:

chain size

Sometimes finding the parent chain isn't always as obvious as it may seem, such as with this compound:

twisted chain

Depending on where you start you may come up with several different answers for the parent chain:

different counting

The correct answer is the chain that is the longest, so in this case the 8 carbon chain. So make sure you pay attention and find the actual longest chain.

Alkanes, Alkenes, and Alkynes

Now that we know how to identify the longest chain and how to name it, we need to realize that this name is only a prefix. Even if we simply have a straight chain of 5 carbons:

pentane

This isn't simply named 'pent' that is only the first part of the name. Now we need to identify if there are any double bonds, triple bonds, or if they are all single bonds. A single bond is an alkane, a double bond is an alkene, and a triple bond is an alkyne. The suffix for alkanes is -ane, alkene is -ene, and alkynes are 'yne'.

In the case above, all of the bonds are single bonds, so it is an alkane. So a 5 carbon chain alkane is called pentane.

Now let's add a double bond in there:

double bond

So this is now a pentene. But now we need to identify where that double bond is located. So we say it is 2-pentene. Since the double bond starts on the second carbon, if we start counting from the left side. Why isn't is a 3-pentene, why don't we start counting from the right side? We always start counting from the side that will give us the lowest number, so we end up with a 2-pentene. Either way we can correctly draw the structure, but traditionally, we use the lower number. The same process is used for alkynes.

Functional Groups

Remember the carboxylic acid that was in the ethanoic acid. Groups such as carboxylic acids are referred to as functional groups. Each group has its own prefix and suffix. Sometimes the prefix will be used, and sometimes the suffix will be used.

functional groups table

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